Abstract. H. Sato introduced a Schwarzian derivative of a contactomorphism of R 3 and with T. Ozawa described its basic properties. In this note their construction is extended to all odd dimensions and to non-flat contact projective structures. The contact projective Schwarzian derivative of a contact projective structure is defined to be a cocycle of the contactomorphism group taking values in the space of sections of a certain vector bundle associated to the contact structure, and measuring the extent to which a contactomorphism fails to be an automorphism of the contact projective structure. For the flat model contact projective structure, this gives a contact Schwarzian derivative associating to a contactomorphism of R 2n−1 a tensor which vanishes if and only if the given contactomorphism is an element of the linear symplectic group acting by linear fractional transformation. §1. IntroductionThe classical Schwarzian derivative is a cocycle, S(f ), of the diffeomorphism group of the real line with coefficients in the quadratic differentials and which vanishes when restricted to the group of projective transformations. It arises naturally in the context of flat projective structures on one-dimensional manifolds and may be interpreted as describing how a normalized second order linear differential operator transforms under a change of variable, provided the operator is viewed as acting on −1/2 densities, rather than on functions (this determines the relevant normalization). Its characteristic properties are: